Concatenating videos (mp4) in Mac
Quicktime 7 Pro is able to concatenate videos.
Alternatively, you can write a SMIL file describing a movie composed of a sequence of your videos:
<smil>
<head>
<layout>
<root-layout width="640" height="480" background-color="gray" />
<region id="main" width="100%" height="100%" fit="fill" />
</layout>
</head>
<body>
<seq>
<video region="main" src="myVideo001.mov" />
<video region="main" src="myVideo002.mov" />
<video region="main" src="myVideo003.mov" />
<video region="main" src="myVideo004.mov" />
</seq>
</body>
</smil>
SMIL is a format readable by Quicktime.
Since those youtube videos probably come in a codec Apple software understands, you could open iMovie (comes with every mac), drag the mp4 files to the program and then concatenate them, with the addition of being able to edit the clips, adjust their sounds, etc.
- Start with an empty project in iMovie.
- Drag one file into iMovie.
- Drag another one into iMovie.
- Now that both are imported, you can arrange them in the timeline of your project.
Check the iMovie manual for more information on how to do this.
The problem with iMovie is that during importation the files are recompressed into Apple's intermediate codec, exploding the required file size for the project (tends to go into the GB range).
Similar software to what has already been suggested is MPEG Streamclip, which allows you to open a video, select all, copy it to the clipboard, then open another one and paste it. It also allows minimal edition features withour requiring intermediate recompression of the source files, but the interface is not polished and easy to use as iMovie.
I wanted to add how to do it with recent Quicktime Versions (i. e. the one shipped with Lion), so users who find this thread looking for a solution (like I was) find a current answer.
Just open the first clip and select from the menu: "EDIT" -> "Add clip at the end..." (or so, my menu is German). Select the movie you wanna add. Repeat for the other parts.
Then select "FILE" -> "Export". This might take a while and can change the encoding of your videos, but it works without installing anything and is pretty easy.